(1) Field of the Invention
The invention relates to a method of making a sandwich lightweight construction material.
(2) Description of the Prior Art
Lightweight constructional building materials are used in all cases where a design saving material is important, i.e. a low weight. Lightweight construction materials are therefore mainly employed in aircraft or vehicle construction but are also used in structural engineering. Apart from the saving in weight however, the mechanical stability, i.e. the strength, loadability and resistance to torsion are further essential demands made of lightweight construction materials.
The necessary mechanical stability is employed by the socalled sandwich construction in which two thin loadbearing cover layers are employed which are joined together via a thicker but lighter core material.
Depending on the field of use these covering or top layers consist of metal, plywood or fibre-reinforced synthetic materials; as fillers, hard foams, balsa wood or honeycomb cores of a great variety of materials are used as well as filigrane materials with network structure. Said honeycomb or network structure is made from warp knit fabrics impregnated with synthetic resin by deep drawing.
It is further known to employ nonwoven fabric or needle felt of high pore volume as core material.
With this method of construction weight-related strengths can be achieved which are several times greater than the values of steel. However, these structures do not always meet all the requirements as regards fire behaviour, deformability, strength and toxic behaviour.
A further disadvantage resides in that the core material must be joined to the cover layers for example by adhering so that in the manufacturing of the sandwich component an additional working operation is necessary. The nature of the joining of the core material and cover layers also decisively influences the mechanical stability of the entire construction component so that the requirements cannot be met frequently precisely as regards the resistance to torsion and twisting. This is the case in particular when a honeycomb structure is used as core material which as binding area has only the thin webs of the honeycomb walls perpendicular to the covering layer.
U.S. Pat. No. 4,336,296 discloses a three-dimensional fabric which to increase the stability is dipped into a plastic composition which serves to stick the individual filaments together. In this manner a flexible structure is obtained which is used for making vehicle tires. This fabric cannot be employed as lightweight construction material.
GB-PS NO. 990,554 discloses a two-layer fabric in which the two layers are joined together via pile threads. For example, to obtain stable boards this double fabric is impregnated with a thermosetting synthetic resin which sets in the subsequent heat treatment. A disadvantage here is the additional working step of the impregnation.
U.S. Pat. No. 3,670,504 describes a double fabric of polymers in which the pile threads are spaced apart far enough for the double fabric to be filled with concrete. It is the filling material which imparts stability to the fabric.
U.S. Pat. No. 3,207,185 discloses boards which consist of a double fabric and in which the joining between the upper and lower fabrics does not consist of through individual threads but of the same fabric as the covering layers. Special machines are therefore required for the manufacture. For consolidating the fabric made from polymers, said fabric is also impregnated with a synthetic resin which subsequently sets in a heat treatment.